If you're not able to download Minecraft properly and you're seeing an crash report that looks like the below error message, it's because of ByteFence, which is known to cause problems with Minecraft. The only way to fix it is to uninstall it.

A typical error looks like this:

Error details: Attempting to move successful download to C:/Users\User\AppData\Roaming\.minecraft\assets\objects\somethingsomethingsomething The process cannot access the file because it is being used by another process

Over at #minecrafthelp we've received thousands of reports of people not being able to download Minecraft because of this. If you're having the same problem, check if you have ByteFence installed. (Other antiviruses can also cause this – try uninstalling it and play MC to see if problem persists.)

(If you need more help, hop on #minecrafthelp and we'll take a look at your computer.)